1. Field of the Invention
The present invention relates generally to a system and method for controlling an engine wherein a value correlative to an output generated from the engine is detected and an engine control parameter such as an engine throttle valve, an opening angle of which determines an intake air quantity, is actuated so that the detected value reaches a target value, preferably wherein in addition to the actuation of the throttle valve to control intake air quantity an air-fuel mixture ratio is controlled so that the engine output reaches near a limit value in a stable region previously set, and preferably wherein in addition to the actuation of the throttle valve an ignition timing is so controlled that the engine output reaches a maximum.
2. Description of the Prior Art
In a conventional gasoline engine, an accelerator pedal is mechanically linked with a throttle valve installed within an engine intake manifold. An angular position of the accelerator pedal (when an operator depresses the pedal) determines an opening angle of the throttle valve, i.e., an intake air quantity supplied to the engine, so that the engine output is directly controlled.
In detail, in a conventional electronically controlled engine, an optimum quantity of fuel injected into the engine is previously set according to the intake air quantity thus determined, an engine speed, various engine operating parameters such as engine cooling temperature. A plurality of fuel injection valves, each located within a corresponding intake port of the corresponding engine cylinder, which is located downstream of the throttle valve, inject fuel wherein the quantity of fuel is determined by the optimum quantity of fuel previously set, so that a mixture of fuel with intake air is formed and supplied to the engine body. Thus, the engine operation may be controlled.
Since in such conventional engine control systems the quantity of fuel supplied to the engine is controlled on the basis of the actual quantity of intake air determined according to the opening angle of the throttle valve, i.e., the angular position of the accelerator pedal, the engine output is varied even with the same throttle valve opening angle depending on the difference of environment under which the engine is placed (, e.g., atmospheric pressure and surrounding air temperature of the engine).
For example, when the above-described engine is placed on high ground, i.e., under a low atmospheric pressure, intake air quantity is reduced so that the quantity of fuel injected into the engine is accordingly reduced. Thus the engine output is reduced, even though the opening angle of the throttle valve is the same as that when the engine is placed on low ground, i.e., under a normal atmospheric pressure.
Therefore, it is necessary to depress the accelerator pedal through an angle greater than that when the engine is placed at low ground in order to achieve the same engine output as that when the engine is placed at low round. In this way, engine driveability is lowered.
In a similar way, the variation in the engine output is caused by aging effects even though the opening angle of the throttle valve is the same so that the engine driveability will be lowered.
Furthermore, since a deviation of the engine output from a target value means that the engine output is deviated from a stable region in which the engine carries out a stable operation, the engine cannot provide a sufficient engine output and it results in a wasteful consumption of fuel when the engine output is out of the stable region.